The tip of my Apple Pencil fell off and now it doesn’t work.

Has just the replaceable plastic Pencil Tip detached from the Pencil - or has the fragile metal transducer that is covered by the plastic tip been broken or damaged?

If the transducer has been damaged or broken, as will often happen if the Pencil is dropped onto its tip, then irreparable damage will have occurred - requiring replacement of the Pencil itself.

If only the plastic Pencil Tip is damaged, replacements can be purchased directly from Apple:

Apple Pencil Tips - 4 pack

https://store.apple.com/uk/xc/product/MLUN2ZM/A

Apple Pencil Replacement tips

https://www.apple.com/shop/product/MLUN2AM/A/apple-pencil-tips-4-pack?fnode=17d94476e4252fc202a1082a5e623e903abdbaa2dd795251a37c5dd38f61db56059862fb118e9a3f15a66cafac7e3400c06271e56890193ee778806965082afde016ac9a39e210b348be1ef010c203bfb070fd2e43769f40475d677c447299bb24bdcf3cfb4ea098d913f5f76b61d23f

The Apple Pencil is not repairable. As the fragile transducer covered by the replaceable pencil tip will be damaged or broken, the only remedy is total replacement of the Pencil.